Space industry insiders have a love-hate relationship with Ron Howard’s 1995 film Apollo 13. On one hand, the movie depicted many aspects of NASA’s Apollo program with an extremely high degree of accuracy, and it did so in a positive light. The film also helped elevate the Apollo 13 mission and the tremendous engineering work that occurred during its seven days to near-mythical status.

On the other hand, Apollo 13 the film is rife with Hollywood-isms—mostly focused around "amping up" the interpersonal relations between characters. Astronauts Jim Lovell, Jack Swigert, and Fred Haise never raised their voices or yelled at each other on the far side of the moon; no one was ever worried about Swigert’s piloting abilities; and grounded astronaut Ken Mattingly didn’t actually lock himself in a simulator and personally devise a solution for the spacecraft’s power issues.

In the film, the tale of the third lunar landing starts out prosaically enough: Kennedy’s challenge of landing a man on the moon by the end of the 1960s has been met, and the public is no longer excited by space travel. TV coverage for the mission is nonexistent. But on the mission’s third day, at 9:07pm CST on April 13, 1970, something happens that causes the world’s attention to laser-focus on this little spacecraft and its three occupants. A routine maintenance checklist task results in an explosion of one of the spacecraft’s oxygen tanks. Suddenly, the lives of the astronauts are at risk.

The original crew of Apollo 13: still Lovell on the left and Haise on the right, but with Ken Mattingly as the Command Module Pilot. Mattingly was grounded as a precaution after exposure to measles, and he replaced Swigert from the mission's backup crew.

Jim Lovell cuts and serves pieces of his birthday cake in March, 1970. Note the red "commander" stripes on his suit, added after Apollo 12 so that the commander and LMP could be more easily distinguished in photographs on the lunar surface.

So contrary to a now 20-year-old film, what happened exactly? Why did the oxygen tank explode, and how could a defective oxygen tank make it aboard a functioning, healthy spacecraft in the first place? But most important of all, in an age of slide rules and shockingly primitive computers 45 years ago, how did Apollo 13 make it back home after sustaining such crippling damage?

To answer all those lingering questions, we asked an expert: Apollo flight controller Sy Liebergot. He manned the EECOM console for a number of flights—including Apollo 13.

What happened on the ground

During the coast from the Earth to the Moon, the "Apollo spacecraft" was composed of three (actually four) distinct segments: the stubby Apollo Command Module that housed the astronauts, the cylindrical Apollo Service Module that contained the majority of the spacecraft’s electrical and environmental supplies, and the gawky Lunar Module (which was itself made up of two independent stages—the descent stage and the ascent stage).

The Apollo spacecraft used a combination of batteries and three large fuel cells to provide electricity for all its various systems, with the fuel cells generating most of the electricity during the flight. The fuel cells worked by combining hydrogen and oxygen in a reaction that produced 1.4kW of 30-volt DC electricity and potable water. The electricity powered the spacecraft’s systems, and the water was used both for cooling the spacecraft’s systems and also by the crew for drinking, hygiene, food re-hydration, and various other things.

The most efficient way to carry large quantities of hydrogen and oxygen on a spacecraft is in liquid form, and so the Apollo Service Module had two large liquid hydrogen tanks (which each held 13kg of liquid hydrogen) and two somewhat smaller liquid oxygen tanks (which in spite of their size each held 148kg of the much denser liquid oxygen).

Apollo 13’s big problem centered around the second of the two oxygen tanks—called, appropriately enough, "tank no. 2." The spherical tank had been manufactured years earlier by Beech Aircraft under contract to North American Rockwell, and it was originally fitted to the Apollo 10 service module in 1969. Some time before Apollo 10’s launch, the tank was removed from the Apollo 10 service module for maintenance or modification, and it was dropped. It fell from a height of about two inches.

Rather than re-use a potentially damaged tank, another was fitted to Apollo 10. Meanwhile, the dropped tank was inspected and no damage was found. However, the external inspection missed one red flag. Internally, a fill line suffered slight damage.

A beautiful shot of Apollo 15's Command Module, Endeavour and its attached service module.

A diagram illustrating the post-Apollo 13 liquid oxygen tank improvements made as a result of the Apollo 13 accident.

NASA

NASA assigned the seemingly undamaged tank to fly in Apollo 13’s service module. Extensive testing took place again prior to launch, and during one test, the tank couldn’t be properly purged of liquid oxygen (this was done by feeding gaseous oxygen into the tank to push the liquid oxygen out; the damaged fill line made that impossible). The testing team decided to empty the tank by heating it up and forcing the liquid oxygen to boil off.

The tank’s heater—normally used to keep the tank’s temperature and pressure elevated to facilitate the flow of oxygen—had been designed to accept power from the spacecraft’s 28-volt DC system, but it was connected to the ground’s 65-volt DC system for eight hours. The high-voltage current welded the heater switches closed, preventing automated shut-off, and the temperature in the tank rose to more than 1,000 degrees Fahrenheit. The tank’s internal thermometer could display a maximum temperature of only 80 degrees Fahrenheit. Nothing external indicated a problem.

This overnight bake-in did the job of emptying the tank, but it also caused an unknown amount of damage to the tank’s internals. A NASA report suggests that "serious damage" was done to the Teflon insulation coating the tank’s internal wiring.

Lee Hutchinson
Lee is the Senior Technology Editor at Ars and oversees gadget, automotive, IT, and culture content. He also knows stuff about enterprise storage, security, and manned space flight. Lee is based in Houston, TX. Emaillee.hutchinson@arstechnica.com//Twitter@Lee_Ars